This invention relates to video apparatus such as television receivers and video tape recorders, and particularly to a gray level correction device or contrast corrector for video signal which can correct the luminance and color saturation of a video signal so as to make the displayed image more clear and vivid.
Recently, as the large-screen type television receiver becomes popular, a gray level correction device such as the black level detection circuit and gamma correction circuit has often been used for the video luminance signal in order that the image may be seen clear and vivid.
FIG. 3 is a block diagram of a conventional gray level correction device for video signal. In FIG. 3, there are shown a pedestal clamp circuit 1 for clamping a video luminance signal input a to a pedestal level d, and a black level expansion circuit 2 for expanding the signal component level of a dark portion of a clamped luminance signal f in the pedestal level direction when a detected black level i is compared with the pedestal level d and as a result, higher (brighter) than the pedestal level d. In addition, shown at 3 is a gamma correction circuit which corrects the luminance in accordance with the characteristic shown in FIG. 4. That is, the gamma correction circuit 3 compares a detected white level j and a white level control voltage e, and when the detected white level j is not higher than the white level control voltage e, it exhibits the luminance conversion characteristics (reversed gamma characteristics) as shown in FIG. 4A to a clamped luminance signal g with expanded black level, thus correcting it to expand the luminance of the bright portion. When the detected white level j is higher than the white level control voltage e, the gamma correction circuit 3 exhibits the luminance conversion characteristics (gamma characteristics) as shown in FIG. 4B to the clamped luminance signal g with expanded black level, thus correcting it to suppress the luminance of the bright portion. Shown at 4 is a color saturation control circuit for controlling the saturation of a chrominance signal input b in accordance with a color saturation control voltage p so as to produce a chrominance signal output C. This chrominance signal may be color carriers, color differences values or red, green, blue (RGB) video signals. Shown at 5 is a black level detection circuit for detecting the level of the darkest portion at each effective video signal period of the black expanded clamped luminance signal g. Shown at 6 is a white level detection circuit for detecting the level of the brightest portion within the effective video signal period of an output luminance signal h.
The operation of the above-mentioned gray level correction device for video signal will be described below.
First, when the luminance signal input a is supplied to the pedestal clamp circuit 1, this circuit clamps the pedestal of the luminance signal a to the pedestal voltage d, thus producing the clamped luminance signal f. This signal f is supplied to the black level expansion circuit 2. When the detected black level i is higher than the pedestal level d (brighter), the black level expansion circuit expands the luminance of the black portion of the clamped luminance signal f in the pedestal direction, thus producing the clamped luminance signal g with expanded black level. This signal g is supplied to the black level detection circuit 5, which then detects the black level of the effective luminance signal. Moreover, the signal g is also supplied to the gamma correction circuit 3, where the white level control voltage e and the detected white level j are compared and the gray correction is made, thus the corrected luminance signal h being produced. The corrected luminance signal h is also supplied to the white level detection circuit 6, which then detects the white level of the effective luminance signal. On the other hand, independently of the above correction of the luminance signal, the color saturation control circuit 4 receives the chrominance signal b and controls its color saturation in accordance with the color saturation control voltage p so as to produce a controlled chrominance signal c.
In the above arrangement, however, only the luminance signal is corrected for the black level and is gamma-corrected without any affection to the chrominance signal. Therefore, when the luminance signal is corrected for the gray level, the luminance and the chrominance signals of that portion are unbalanced with the result that the reproduced color is seen different from the original color.